1. Field of the Invention
This invention relates generally to processes for texturing data storage disk substrates and specifically to a highly controllable laser texturing process for brittle nonmetallic surfaces such as a glass substrate for a data storage disk.
2. Description of the Related Art
Recent magnetic disk drive designs employ a commonly denominated Contact Start-Stop (CSS) system in which the magnetic head contacts the magnetic disk surface when the disk is stationary. When the disk starts to rotate, the magnetic head slides along the surface, eventually flying fully lifted from the surface because of laminar air flow at the disk surface.
A smooth, specular recording surface is preferred in the art to permit the magnetic head to ride as closely as possible to the disk surface. Creation of a "textured" region on the metallic disk surface for magnetic head contact during CSS is used in the art to overcome the excessive contact stiction and friction developed during startup and stopping of disk rotation. The magnetic head is moved to the "CSS region" at the appropriate times by the drive controller. The specular smoothness of the remainder of the disk surface is retained to permit high-density magnetic data recording.
Practitioners in the art have proposed several useful techniques for texturing metal disk surfaces. For instance, the use of repetitive laser pulses to create reproducible pits on a metal surface is known in the art for application to sheet-metal stamping surfaces, liquid transfer surfaces and metal data storage disk surfaces, as discussed in the above-cited co-pending patent application. Unfortunately, such techniques are not generally useful for texturing of brittle nonmetallic disk substrate surfaces such as the glass substrate known in the art for certain data storage disk applications. A brittle nonmetallic surface, especially a glass surface, is expected to crack or deform excessively during the grinding or laser texturing techniques known in the art for metal disk substrates. Accordingly, chemical surface texturing techniques and other patterned deposition techniques are preferred in the art for brittle surfaces such as ferrite or glass.
For instance, in U.S. Pat. Nos. 5,079,657 and 5,162,073, Michael I. Aronoff et al. disclose a selective chemical etching technique for texturing the flying surface of a magnetic head. Aronoff et al. teach a stiction-reducing approach that avoids the necessity of texturing a CSS region on the recording disk but is disadvantageously limited to specific head surface materials.
In U.S. Pat. No. 4,985,306, Toshinori Morizane et al. disclose a method for manufacturing a glass base plate for a recording disk that includes subjecting the base plate to chemical etching treatments that provide different rates of etching between a crystallized material and an amorphous material. Morizane et al. teach the use of their method for creating a textured CSS region in the base plate that is reproduced through a subsequently-deposited recording material layer. Other practitioners have suggested using expensive chemical vapor deposition (CVD) processes to create roughened zones on a glass disk surface before deposition of the magnetic recording film layer.
Some practitioners in the printing arts teach the use of laser pulses to create many tiny pits in the surface of a brittle material such as tungsten carbide. For instance, in U.S. Pat. Nos. 5,143,578 and 5,236,763, Pierre Luthi discloses a method for engraving a series of consecutive cells or pits in a solid surface such as the liquid transfer surface of a printing roller. Luthi recommends his technique to overcome the surface hardness of the ceramic and metal carbide surfaces normally used in engraving applications but neither considers nor suggests solutions to the particular problems of CSS region creation in brittle nonmetallic disk surfaces to overcome head stiction.
Others have considered the specific problem of using laser energy for reducing stiction in data recording disks having glass substrates. For instance, in Japanese patent 4-311814, Maeta Hiroshi discloses a technique for texturing the glass substrate without reducing surface durability by applying a laser pulse from the back of the translucent substrate to fracture and scatter small particles of glass at the front surface. The laser pulse induces thermal shock that effectively breaks up the front surface into fine grains that are then supposedly partially annealed to the front surface, creating a roughened topography suitable for reducing stiction in the magnetic recording film layer that is later added to the front glass substrate surface. Hiroshi doesn't discuss how his method overcomes the problems resulting from uncontrolled surface damage.
In U.S. Pat. Nos. 5,062,021 and 5,108,781, Rajiv Ranjan et al. disclose a process for creating a series of closely-spaced pits in the metallic surface of a magnetic recording disk to reduce stiction. Ranjan et al. teach the use of a flashlamp-pumped neodymium-yttrium-aluminum-garnet (Nd:YAG) laser to create the necessary surface roughness. However, they neither consider nor suggest the application of their texturing method to glass disk substrates or other brittle nonmetallic surfaces, and indeed their method is generally known to be impractical for brittle nonmetallic materials.
Practitioners in the art generally avoid using laser pulses to create controlled changes to the topography of a brittle nonmetallic surface, expecting hairline cracking or surface material damage. This damage, often at the microscopic level, is frequently observed after laser melting of any brittle nonmetallic material such as glass because of the rapid melting and resolidification caused by a short laser pulse. This rapid melting and resolidification usually causes excessive stress in the glass, which forces the surface to crack and break up. Such laser pulses are herein said to create stress that exceeds a "thermal shock threshold" for the brittle nonmetallic surface material. Indeed, the above-cited Hiroshi patent relies on this thermal shock micro-fracturing phenomenon for the usefulness of the claimed invention. Laser pulse energies below this threshold are generally believed to be useless for altering the topography of brittle nonmetallic surfaces.
Accordingly, there is a clearly-felt need in the art for a brittle nonmetallic surface texturing technique with the controllability and other advantages of the laser-texturing techniques known in the art for metal surfaces. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.